Mountain bikes - “MTBs” - whose rear suspension consists of a swinging arm articulated at the bottom part of the seat tube and interacting with a shock absorber are well known. Said swinging arm pivots about a fixed rod parallel to the axle of the drive sprocket carried by the bottom bracket positioned at the lower end of the frame, i.e. at the intersection of the down tube and the seat tube of the frame. Furthermore, the ends of the shock absorber are secured to the swinging arm and, respectively, to the horizontal tube connecting the seat tube to the bike's forks or to an intermediate tube extending globally from the bottom bracket to the horizontal tube, said seat tube being interrupted in order to allow the shock absorber to pass.
When there is an energetic kick on the pedals, such as a kick on the pedal in order to move the bike off or when the cyclist stands out of the saddle, on the pedals, this type of rear suspension generates a “pumping” effect resulting in a cyclical compression of the suspension, even on a perfectly level surface. This type of rear suspension thus has the drawback of dissipating some of the drive torque provided by the cyclist in the shock absorber instead of contributing to driving the bike along.
Also known are mountain bikes - “MTBs” - whose rear suspension consists of a swinging arm articulated on the seat tube and/or on the down tube of the frame by means of two rods or the like, said swinging arm or at least one of the rods interacting with a shock absorber. Said swinging arm thus pivots about a movable virtual pivot point, also called the instantaneous centre of rotation, which corresponds to the intersection of the straight lines passing via the respective axes of the rods. This is the case of patent application US 2002/0109332, for example, which describes such a rear suspension for a bike. The instantaneous centre is displaced in the lower front quadrant and is displaced rearwards and downwards when the swinging arm is displaced upwards when an obstacle such as a bump, for example, is overcome.
Such a rear suspension also has the drawback of generating a “pumping” effect resulting in a cyclical compression of the suspension even on a perfectly level surface, thus dissipating some of the drive torque provided by the cyclist in the shock absorber instead of contributing to driving the bike along.
Similarly, international patent application WO 03/018392, which describes a mountain bike - “MTB” - whose rear suspension consists of a swinging arm articulated on the seat tube by means of two rods or the like, one of the rods - the upper rod - interacting with a shock absorber. Said swinging arm thus pivots about a movable virtual pivot point, also called instantaneous centre of rotation, which corresponds to the intersection of the straight lines passing via the respective axes of the rods and is displaced forwards and downwards when the swinging arm is displaced upwards when an obstacle such as a bump, for example, is overcome.
This rear suspension also has the drawback of generating a particularly detrimental “pumping” effect.
In order to remedy this drawback, an “anti-pumping” MTB rear suspension has already been devised, as described in French patent FR 2 774 966. Such an MTB includes a frame, a swinging arm secured to the frame carrying the axle of the hub of a drive wheel and a shock absorber whose ends are secured, respectively, to the frame and to the swinging arm, the drive torque being transmitted to the drive wheel by a chain extending between a drive sprocket secured to the frame and a driven sprocket secured to the axle of the hub of the drive wheel, said chain between the drive and driven sprockets forming two strands, a tensioned upper strand that transmits the drive torque to the drive wheel and another, lower “return” strand. The swinging arm is secured to the frame by two rods, a first rod that connects the front end of the swinging arm to the down tube of the frame, said rod extending globally vertically, and a second rod that connects said swinging arm in its central part to the seat tube of the frame, said second rod extending globally horizontally. The instantaneous centre of rotation about which the swinging arm pivots, corresponding to the intersection of the straight lines passing via the respective axes of the rods, is aligned with the axis of the tensioned strand of the chain when the latter is on one of the chainwheels of the pedals and bottom bracket bearing assembly, when the MTB is in a static equilibrium position, i.e. stationary, straddled by a cyclist of average weight, and is displaced upwards along a globally rectilinear path when the swinging arm pivots upwards, compressing the shock absorber's spring.
Also known is French patent FR 2 827 831, which describes a two-wheel vehicle with an “anti-pumping” rear suspension. The MTB is equipped with a rear suspension and includes a front frame carrying a bottom bracket and a swinging rear assembly carrying a rear drive wheel. The frame and the rear assembly are secured by two pivoting links and by a shock-absorber system. The first pivoting link is formed by a circular cam formed in a casing carrying an eccentric rod, said casing being mounted pivotably in a housing on the front frame. The rear assembly is fixed on the eccentric rod, on either side of the casing. The second pivoting link is formed by a rod consisting of a simple plate arranged in the general direction of the frame. In the same manner as previously, the instantaneous centre of rotation about which the swinging rear assembly pivots is displaced vertically along a globally rectilinear path.
Lastly, there is French patent FR 2 821 603, which describes an anti-pumping system for a wheeled vehicle, the drive torque of which is transmitted by means of a transmission chain to the drive wheel carried by a swinging arm. This MTB includes a triangular frame, a swinging arm secured to the frame and a shock absorber whose ends are secured, respectively, to the frame and to the swinging arm. Said swinging arm is secured to the frame by two rods extending globally perpendicularly with respect to one another, one of the rods extending globally vertically when the vehicle is in a static equilibrium position. The intersection of the straight lines passing via the axes of rotation of each rod forms the instantaneous centre of rotation about which the swinging arm pivots and is located in the upper rear quadrant of a reference system defined by a horizontal axis and a vertical axis convergent with the axis of the bottom bracket. This instantaneous centre of rotation is located on the upper strand of the chain or on the straight line extending it in a static equilibrium position and said instantaneous centre of rotation is displaced globally vertically along a straight line when the swinging arm pivots upwards.
All these devices, although generating an anti-pumping effect, have the drawback of resulting in a lengthening of the chain strand when the swinging arm is displaced upwards, which gives rise to a variation in chain tension that generates a kick-back effect that counteracts rotation of the pedals and bottom bracket bearing assembly. This kick-back effect gives rise to a disagreeable, jolting sensation for the cyclist.
In order to remedy the lengthening of the chain strand that gives rise to a kick-back effect when the swinging arm is displaced upwards when an obstacle is overcome, U.S. Pat. No. 5,553,881, U.S. Pat. No. 5,628,524, U.S. Pat. No. 6,206,397 and U.S. Pat. No. 6,488,301 are well known, which describe a suspension system for a bicycle. The bicycle includes a compressible rear suspension and a chain transmission that comprises a pedal-actuated drive chain wheel and a rear-wheel hub, said transmission having a chain-strand length that is the distance, in the rearward direction, from the axis of said pedal-actuated drive chain wheel to the axis of said rear-wheel hub, which is represented by a variable value CSL, and said suspension having means for displacing said rear-wheel hub in a direction oriented substantially upwards along a controlled wheel displacement path in response to a compression of said suspension, in which the position of said hub along said path from a predetermined starting point at which said suspension is in extension is represented by a variable value D that increases with the increase in the compression of said suspension. Said means give rise to a pivoting point, called the instantaneous centre of rotation or virtual pivoting point, defining the displacement path of the rear wheel such that said displacement path includes a preferred pedaling position at a predetermined position Dp located along the rear-wheel displacement path, a lower curve segment extending substantially below said position Dp, in which the rate of lengthening of the chainstay increases with the increase in compression of said suspension system, such that the first derivative relationship d[CSL]/d(D) is a curve having a generally positive slope, so that the second derivative relationship d2[CSL]/d(D))2 is generally positive, and an upper curve segment, curved upwards and forwards, extending substantially above said position Dp, in which the rate of lengthening of the chainstay decreases with the increase in compression of said suspension system, such that the first derivative relationship d[CSL]/d(D) is a curve having a generally negative slope, so that the second derivative relationship d2[CSL]/d(D))2 is generally negative. Preferably, the lower curve segment of said rear-wheel displacement path comprises a first lower arc segment having an average radius extending forwards that is greater than an arc of constant radius from said axis of the drive chainwheel to said axis of the hub of the rear wheel and an upper arc segment having an average radius extending forwards that is less than said radius of said first lower arc segment, said lower arc segment joining said upper arc segment at a point of inflection close to said position Dp, such that a part of the lower curve segment has a general S shape and, furthermore, the rate of lengthening of the chain strand has a peak when D reaches and is displaced above said position Dp.
This type of bike rear suspension has the drawback, although it limits the kick-back effect, of not generating an anti-pumping effect, the instantaneous centre of rotation being displaced globally forwards and downwards when the swinging arm is displaced upwards.